One goal of optical spectroscopy is to determine the spectral content of electromagnetic radiation after it has interacted with some material or sample of interest. Typical wavelength dependent interactions include reflection, scattering and absorption and transmission.
Common instruments in the prior art fall into two general classes: spectrographs and spectrometers. A spectrograph disperses a spectrum in one step and records it using a multichannel optical detector, e.g. a photodiode array or a CCD camera. A spectrometer, in contrast, scans the spectrum mechanically or electronically and records the response sequentially using a single optical detector (D. W. Ball, “Field Guide to Spectroscopy”, SPIE Press, 2006).
Realizations of spectrographs typically comprise dispersive elements, e.g. prisms, reflection or transmission gratings, or arrayed waveguide gratings. Current realizations of spectrometers are usually based on the superposition of light, e.g. by using Michelson-, Fabry-Perot-, or Mach-Zehnder couplers or interferometers.
Current advanced spectrometers can separate several 1000 channels with a spectral resolution in the order of 1 nm. Fourier transform infrared spectrometers (FT-IR) based on Michelson interferometers relax the requirements on the dimensions of the entrance slits and therefore achieve higher output signals (Jacquinot principle). In order to further increase the signal-to-noise ratio, multiple scans are employed (Fellgett principle). The spectral resolution, however, is related to the optical path difference and thus to the span of the moving mirror. Thus, the mechanical stability and the reliability of high-resolution instruments represent an important cost-factor which restricts the application of FT-TR spectrometers to high-end laboratory equipment. In addition, due to the sequential measurement mode, they suffer from measurement periods in the order of minutes for high-resolution spectra.
The measurement period of spectrographs, in contrast, is only limited by the response time of the multichannel optical detector and the subsequent electronic circuitry. Many spectrographs are mechanically robust since they do not exhibit any moving parts. Generally, their spectral resolution is limited by the number of equally spaced wavelength channels. Although this is not a physical limit, shifting it causes rapidly increasing technical difficulties and costs. Furthermore, compared to FT-IR spectrometers, spectrographs are more sensitive to the thermal noise of the detectors.